Piezoelectric elements are especially (but not exclusively) used as actuators or control elements. Piezoelectric elements can be so used because they are known to contract or expand in response to the voltage applied to them.
The practical realization of control elements using piezoelectric elements has proven particularly advantageous when a particular control element has a task of a rapid execution and/or frequent movements.
It is also advantageous to use piezoelectric elements, for example, as a control element in fuel injection nozzles for internal combustion engines. European Patent Application Nos. 0 371 469 and 0 379 182 describe such application.
Piezoelectric elements are capacitive consumers, which, as described above, contract or expand in response to a particular charging state or in response to their voltage rise or the voltage being applied to them.
Two conventional principles are known for charging and discharging a piezoelectric element. In particular, charging and discharging of the piezoelectric element via an ohmic resistor and charging and discharging of the piezoelectric element via a coil. The ohmic resistor and the coil limit the charge current that occurs during charging and the discharge current that occurs during discharging.
The first conventional arrangement for charging and discharging a piezoelectric element 101 via an ohmic resistor is illustrated in FIG. 9.
The piezoelectric element 101 is connected to a charging transistor 102 and to a discharging transistor 103.
Charging transistor 102 is driven by a charging amplifier 104 and, in a connected-through state, charging transistor 102 connects piezoelectric element 101 to a positive supply voltage; discharging transistor 103 is driven by a discharging amplifier 105 and, in the connected-through state, discharging transistor 103 connects piezoelectric element 101 to ground.
In the connected-through state, a charge current flows through the charging transistor 102 for charging piezoelectric element 101. With the increasing charge of piezoelectric element 101, the voltage arising on piezoelectric element 101 rises, causing its external dimensions to also change, correspondingly. The blocking of charging transistor 102, i.e., interrupting or ending the charging process, results in the charge stored in piezoelectric element 101 or the voltage arising at piezoelectric element 101 as a result, and thus the active external dimensions of piezoelectric element 101 are maintained essentially unchanged.
In the connected-through state, a discharge current flows through discharge transistor 103 for discharging piezoelectric element 101. As piezoelectric element 101 is increasingly discharged, the voltage at piezoelectric element 101 drops, and its external dimensions also change, correspondingly. The blocking of discharging transistor 103, i.e., interrupting or ending the discharging process, results in the charge still being stored in piezoelectric element 101 or the voltage at piezoelectric element 101 as a result, and thus the active external dimensions of piezoelectric element 101 are maintained essentially unchanged.
Charging transistor 102 and discharging transistor 103 act as controllable ohmic resistors for the charge current and for the discharge current, respectively. The controllability of the charge current and of the discharge current makes it possible to exactly control the charging process and discharging process. However, the charge current flowing through charging transistor 102 and the discharge current flowing through discharging transistor 103, in this case, produce substantial power losses. The energy loss occurring in the transistors is at least twice as high, per charge-discharge cycle, as the energy stored in piezoelectric element 101. This high energy loss causes a very extensive heating of charging transistor 102 and discharging transistor 103, which is a considerably disadvantageous.
A second conventional variant described above for charging and discharging the piezoelectric element (i.e., charging and discharging via a coil) is also utilized, which is illustrated in FIG. 10.
The piezoelectric element to be charged or discharged, designated in FIG. 10 with the reference numeral 201, is a component part of a charging circuit which can be closed by a charging switch 202 and of a discharging circuit which can be closed by a discharging switch 206, the charging circuit is a series circuit including charging switch 202, a diode 203, a charging coil 204, piezoelectric element 201 and a voltage source 205. A discharging circuit is a series circuit including discharging switch 206, a diode 207, a discharging coil 208, and piezoelectric element 201.
Diode 203 of the charging circuit prevents a current discharging the piezoelectric element from flowing in the charging circuit. Diode 203 and charging switch 202 can be produced together as a semiconductor switch.
Diode 207 of the discharging circuit prevents a current charging the piezoelectric element from flowing in the discharging circuit. Diode 207 and charging switch 206, similarly to diode 203 and charging switch 202, can be produced together as a semiconductor switch.
If charging switch 202 (which is usually open) is closed, a charge current flows in the charging circuit charging piezoelectric element 201. After piezoelectric element 201 is charged, there is essentially no change in the charge stored in piezoelectric element 201 or in the voltage arising at piezoelectric element 201, and thus also in its active external dimensions.
If discharging switch 202 (which is usually open) is closed, a discharge current flows in the discharging circuit discharging piezoelectric element 201. After piezoelectric element 201 is discharged, there is essentially no change in its charge state or in the voltage arising at piezoelectric element 201, and, therefore, also in its active external dimensions.
Charging coil 204 and discharging coil 206 represent an element having an inductive effect on the charge current or the discharge current, respectively. Charging coil 204 and piezoelectric element 201, as well as discharging coil 206 and piezoelectric element 201, constitute an LC-series resonant circuit during the charging and discharging of a piezoelectric element.
Circuits described above with reference to FIG. 10 and methods for charging and discharging the piezoelectric elements using such circuits are described in European Patent Application Nos. 0 371 469 and 0 379 182.
With the circuits described above with reference to FIG. 10, neither the charging circuit nor the discharging circuit has any significant ohmic resistors, and the thermal energy produced by the charging and discharging of the piezoelectric element (and by the flow of the charge current and the discharge current through ohmic resistors) is extremely small.
However, to practically produce such circuits, particularly due to a considerable size of charging coil 204 and discharging coil 208, a relatively large amount of space is required. Thus, in certain cases, the charging and discharging of piezoelectric elements via an element having essentially an inductive effect on the charge and discharge current is not easily achievable or not even possible.
One of the objects of the present invention is to efficiently charge and discharge the piezoelectric elements, even in cramped spaces and conditions.